Scientific Scientific Serendipity:Serendipity:

Building Planets in Plastic Building Planets in Plastic BagsBags

Article: Building Planets in Article: Building Planets in Plastic BagsPlastic Bags

APRIL 13, 2004APRIL 13, 2004

by David Tytellby David Tytell

Adapted by W. Sturges 2010Adapted by W. Sturges 2010

Sometimes important scientific Sometimes important scientific discoveries happen by chance:discoveries happen by chance:

In 2003 astronaut Donald R. Pettit In 2003 astronaut Donald R. Pettit (NASA/Johnson Space Center) was (NASA/Johnson Space Center) was aboard the International Space Station aboard the International Space Station preparing his weekly Saturday Morning preparing his weekly Saturday Morning Science program, in which he Science program, in which he performed various experiments performed various experiments highlighting the fun things one can do highlighting the fun things one can do in microgravity. in microgravity. Before him were plastic bags Before him were plastic bags containing salt, sugar, and coffee containing salt, sugar, and coffee grounds. The demonstration he had grounds. The demonstration he had planned that morning was fairly planned that morning was fairly mundane - shake the bags and watch mundane - shake the bags and watch what happens. what happens. Little did Pettit know he might be about Little did Pettit know he might be about to solve experimentally one of most to solve experimentally one of most perplexing paradoxes surrounding the perplexing paradoxes surrounding the formation of planets.formation of planets.

The problem:The problem:

For decades theorists have had For decades theorists have had trouble growing planets starting trouble growing planets starting from small dust grains in a from small dust grains in a protoplanetary gas-and-dust disk.protoplanetary gas-and-dust disk.

Given a turbulent disk Given a turbulent disk environment with high winds and environment with high winds and high-velocity (100 meters/second) high-velocity (100 meters/second) impacts between objects, small, impacts between objects, small, millimeter-size clumps should millimeter-size clumps should have difficulty growing to have difficulty growing to centimeter size and larger without centimeter size and larger without breaking back into millimeter breaking back into millimeter fragments. fragments.

No one had ever seen it work No one had ever seen it work experimentally.experimentally.

What Pettit did:What Pettit did:

Pettit proceeded to take the bags Pettit proceeded to take the bags of particles and shake them in of particles and shake them in front of the camera. front of the camera.

Within seconds, as the videos Within seconds, as the videos show, the grains clumped together show, the grains clumped together to form centimeter-scale to form centimeter-scale structures. structures.

These fractal-like globs appear These fractal-like globs appear particularly strong, colliding with particularly strong, colliding with the walls of the plastic bag and the walls of the plastic bag and remaining largely intact.remaining largely intact.

Small salt grains (0.5 millimeters across) Small salt grains (0.5 millimeters across) clump to form centimeter-size structures clump to form centimeter-size structures

in seconds when left alone in in seconds when left alone in microgravity. microgravity.

Courtesy Donald R. Pettit (NASA/Johnson Space Center)Courtesy Donald R. Pettit (NASA/Johnson Space Center)

When the salt grains clump together, the When the salt grains clump together, the fractal structures are much stronger than one fractal structures are much stronger than one might assume since these structures survive might assume since these structures survive

impacts with the sides of the plastic bag.impacts with the sides of the plastic bag.Courtesy Donald R. Pettit (NASA/Johnson Space Center)Courtesy Donald R. Pettit (NASA/Johnson Space Center)

Someone noticed:Someone noticed:

Fellow astronaut Stanley G. Love Fellow astronaut Stanley G. Love (NASA/JSC) was working spacecraft (NASA/JSC) was working spacecraft communications in Mission Control communications in Mission Control in Houston, Texas when Pettit in Houston, Texas when Pettit began beaming back the images. began beaming back the images. Love, whose training in planetary Love, whose training in planetary science involved asteroids and science involved asteroids and collisions, immediately realized the collisions, immediately realized the implications of this simplistic, gee-implications of this simplistic, gee-whiz demonstration. whiz demonstration. "Don!" Love exclaimed, "Do you "Don!" Love exclaimed, "Do you realize you've just solved the realize you've just solved the middle stage of planetary middle stage of planetary accretion?"accretion?"

The “experiment” was not designed to The “experiment” was not designed to show what was observed !show what was observed !

Both Pettit and Love are the first Both Pettit and Love are the first to admit that the experiment is to admit that the experiment is hardly quantitative science. The hardly quantitative science. The details are far from controlled or details are far from controlled or precise. But, even though some precise. But, even though some quantities may be unrealistic for a quantities may be unrealistic for a protoplanetary disk — such as the protoplanetary disk — such as the particle density and air density — particle density and air density — the effect works for different the effect works for different materials that are both materials that are both homogeneous (salt) and homogeneous (salt) and inhomogeneous (coffee grounds). inhomogeneous (coffee grounds). It also works for particles of It also works for particles of various sizes, from 0.5 mm various sizes, from 0.5 mm through to 6 mm in diameter.through to 6 mm in diameter.

Larger salt grains (1–6 millimeters across) Larger salt grains (1–6 millimeters across) exhibit the same behavior as the smaller exhibit the same behavior as the smaller

particles. particles. Courtesy Donald R. Pettit (NASA/Johnson Space Center)Courtesy Donald R. Pettit (NASA/Johnson Space Center)

A hidden force in planet-building: A hidden force in planet-building: static static electricityelectricity

Pettit and Love have proposed an Pettit and Love have proposed an explanation for the phenomenon — explanation for the phenomenon — electrostatics. electrostatics. When Pettit rubbed his finger on the bag, When Pettit rubbed his finger on the bag, the particles inside were instantly drawn to the particles inside were instantly drawn to the place he poked. the place he poked. While the electric charge on each grain is While the electric charge on each grain is unknown, most experts agree that unknown, most experts agree that electrostatics and charge exchanges electrostatics and charge exchanges between particles occur in protoplanetary between particles occur in protoplanetary nebulae. nebulae. "We know electrostatics are there. There is "We know electrostatics are there. There is definitely charge in nebulae," says Stuart J. definitely charge in nebulae," says Stuart J. Weidenschilling (Planetary Science Weidenschilling (Planetary Science Institute). Institute). Observations like these are "icing on the Observations like these are "icing on the cake" for proving that electrostatics matter cake" for proving that electrostatics matter in planet formation, he says.in planet formation, he says.

By touching the bag, electric charge is imparted By touching the bag, electric charge is imparted onto the system and the salt particles onto the system and the salt particles immediately react. immediately react. This implies that This implies that electrostaticselectrostatics might be the might be the reason for the reactionreason for the reaction.. Courtesy Donald R. Pettit (NASA/Johnson Space Center)Courtesy Donald R. Pettit (NASA/Johnson Space Center)

Static electricity causes Static electricity causes lightning:lightning:

Steven Desch (Arizona State Steven Desch (Arizona State University), also notes that the University), also notes that the idea of electrostatics is hardly far-idea of electrostatics is hardly far-fetched. fetched. He and others have even He and others have even predicted that lightning is possible predicted that lightning is possible in planetary nurseries. We see in planetary nurseries. We see lightning in various dry lightning in various dry environments on Earth such as environments on Earth such as volcanic ash and sandstorms, says volcanic ash and sandstorms, says Desch.Desch.

Can what happened in a plastic bag be Can what happened in a plastic bag be responsible for starting responsible for starting planetplanet formation? formation?

The largest concern about the study is The largest concern about the study is one of scaling: one of scaling: Can salt grains bouncing around in a Can salt grains bouncing around in a Ziploc bag truly represent Ziploc bag truly represent protoplanetary pieces in a primordial protoplanetary pieces in a primordial solar system? solar system? Do the density of particles in the bag, Do the density of particles in the bag, the rate of collisions observed, the the rate of collisions observed, the apparent strength of the centimeter-apparent strength of the centimeter-scale agglomerations, and the charges scale agglomerations, and the charges within the bag approximate ancient within the bag approximate ancient circumstances well enough to say circumstances well enough to say anything about our planet's past? anything about our planet's past? Pettit and Love don't claim to know Pettit and Love don't claim to know the answers — they simply presented the answers — they simply presented the experiment as a way to mobilize the experiment as a way to mobilize theorists around a potential solution.theorists around a potential solution.

Still, the pair can speculate. Still, the pair can speculate. When asked by Love if Pettit could make a When asked by Love if Pettit could make a Jupiter this way, he replied, "I'll need a Jupiter this way, he replied, "I'll need a bigger bag."bigger bag."